[Cake] start at interval handling

[Jonathan Morton]

> On 12 Oct, 2015, at 00:30, Dave Taht <dave.taht at gmail.com> wrote:
> 
> so... anyway, this does start some api breakage, also. Anyone else got
> braincells?

Some of that is definitely wrong.  The rate_overhead parameter needs to be signed, for a start, since some last-mile encapsulations have *less* overhead than Ethernet.  That’s why I had it as ‘short’ instead of ‘u16’.

 - Jonathan Morton